The present application is related to seam-welded, air hardenable steel structures, tubing and pipe, structures created therefrom and methods for manufacturing seam-welded, air hardenable steel constructions.
A seam-welded, air hardenable steel tube substantially free of cracks in the weld zone has not been proposed in the prior art. “Air hardenable steel” is herein defined as steel that reaches a level of hardness sufficient to cause cracking when processed in a prior art roller-forming tube mill. Other steels which are sometimes called “air hardenable” do not reach a comparable level of hardness and therefore do not crack when processed in a prior art roller-forming tube mill; these steels are categorized herein as “partially or non-air hardenable” steels.
An example of a non-air hardenable steel currently used to manufacture seam-welded tubing is 4130 steel (UNS G10220). This steel does not crack when formed in a prior art roller-forming tube mill; however, it must be liquid-quenched after heat-treating to attain a high strength and unfortunately this liquid quenching tends to induce high levels of distortion. As a result, liquid quenched materials like 4130 have limitations when used for applications requiring frame-type structures that must be straight and free from distortion. An example of such an application is a bicycle frame. Theoretically, the highest strength-to-weight ratio would be attained if the parts could be welded together and then heated and liquid quenched as a whole, resulting in a frame with uniformly high-strength throughout all areas. However, liquid quenching an entire frame at one time would distort it beyond acceptable limits. Instead, when made from this material, bicycle frames must be constructed of individual tubes that are hardened prior to assembly and then welded or brazed together. Because welding or brazing causes localized weak areas, thicker tube walls must be used to compensate for the loss of strength and this ultimately reduces the strength-to-weight ratio of the frame. In some applications such as high performance bicycle frames, the tube wall is thickened only in those areas that will be weakened by welding or brazing to minimize the reduction of strength-to-weight ratio. Although saving weight, such measures require expensive extra processing steps such as drawing the tube. In summary, although liquid quenched steels like 4130 fill the immediate need, a great improvement in overall utility, usefulness and economy would be realized if a low-cost tubing capable of producing a structure of a higher strength-to-weight ratio were available.
One example of a partially air hardenable steel used for tube making is 410S (UNS S41008), made available by Allegheny Ludlum of Pittsburgh, Pa. 410S is a low carbon modification of 410 (UNS S41000). The low carbon level (0.08% maximum) of 410S prevents austenite formation upon heating, thereby preventing martensite formation upon cooling. This means that the metal doesn't crack in a prior art roller-forming tube mill, but also that it doesn't harden to a high strength condition. Tubing comprised of 410S lacks the strength needed for high performance load bearing applications.
Another example of partially air hardenable steel tubing is True Temper OX Gold and Platinum series tubing, produced by True Temper Sports, Inc. This is a non-stainless tubing intended for use in expensive bicycle frames that is first roller-formed and seam-welded, and then drawn. Although this steel achieves a high strength without cracking in a prior art roller-forming tube mill, it only does so due to the precise addition of expensive alloying components to prevent the heat-affected-zone (HAZ) from fully hardening on the tube mill. As explained in the company's website at the address http://www.henryjames.com/oxplat.html, these alloy steels are specially formulated to mitigate the difficulties inherent in the welding of air hardenable steel. Modifying the material to prevent cracking results in an expensive, specialty tubing with limited usefulness; for most structural applications, its cost cannot be justified. Rather than resort to the use of expensive alloys, it would be beneficial to create a process that could utilize common, inexpensive, air hardenable steel to produce tubing substantially free of cracks. Such a process would be even more beneficial if the work material could have the corrosion resistant properties of stainless steel.
Through discussion with seam-welded tube and pipe manufacturers in the industry, it is apparent that experimentation with seam-welding tubing of air hardenable steel was abandoned due to the heretofore-insurmountable problem of HAZ cracking. In standard tube production practice on a prior art roller-forming mill, the metal in the weld zone is heated and subsequently cools at a natural rate, which is sufficiently rapid to induce hardening of the material. The axial tensile stresses induced by weld zone shrinkage together with the compressive hoop and tangential stresses induced by the sizing and straightening rollers are therefore acting on material that is in a hard and somewhat brittle state. If the material being formed becomes hard and brittle enough, the weld zone will crack and a sound tube cannot be produced. The prior art provides no indication that efforts in solving this problem were fruitful, if, in fact, any such efforts were made. Perhaps because the applicability of such tubing to a vast range of structural purposes was not envisioned, efforts to solve the problem were either never undertaken or were abandoned, and it became an accepted fact in the industry that only non- or partially air hardenable steels can be successfully roller-formed and seam-welded.
Historically, air hardenable steel has been mainly used in applications that do not require welding. Air hardening steels were first commercially developed for use in cutlery for their high hardness. Common air hardenable steels include martensitic stainless steels. As defined herein, and as understood by those skilled in the art, martensitic stainless steels are essentially alloys of chromium and carbon that possess a body-centered-cubic (bcc) or body-centered-tetragonal (bct) crystal (martensitic) structure in the hardened condition. They are ferromagnetic and hardenable by heat treatment, and they are generally mildly corrosion resistant. As reflected in FIG. 1, martensitic stainless steels have also been defined, and are understood by those skilled in the art, as having a nickel equivalent of between about 4 and 12 and having a chromium equivalent of between about 8 and 15.5, where nickel equivalent is equal to (% Ni+30×% C)+(0.5×% Mn) and chromium equivalent is equal to (% Cr+% Mo+(1.5×% Si)+(0.5×% Nb). Either or both of these definitions are acceptable for practicing the present invention. According to these standard definitions, standard martensitic stainless steels include types 403, 410, 414, 416, 416Se, 420, 420F, 422, 431, and 440A-C.
Air hardenable martensitic stainless steels include a relatively high carbon content compared to other stainless steels (0.15% C maximum in type 410 to 0.75% C maximum in type 440), and between 12 to 18% chromium. This composition results in steel with good corrosion resistance, due to the protective chromium oxide layer that forms on the surface, and the ability to harden via heat treatment to a high strength condition, but one that presents difficulties related to welding. Non-stainless air hardenable steels, which contain very high levels of carbon to allow the formation of a martensitic microstructure upon quenching, and are much more expensive than stainless types, also present difficulties related to welding, and have been primarily used for cutting tools.
Due to air hardenable steel's composition being specially formulated to render it heat treatable by a quench and temper process, it presents some unique problems during welding. The thermal cycle of heating and cooling, which occurs within the confined heat-affected-zone (HAZ) during welding, is equivalent to a quenching cycle. The resulting high carbon martensitic structure produced is extremely brittle in the untempered condition. Cracking of the weld zone can occur for several reasons, including:                Hydrogen induced cold-cracking, due to trapped hydrogen in the distorted BCC martensite crystal structure. Tensile stress applied to the weld increases the risk of cracking.        Thermal induced stresses, due to the heat input during welding, degree of joint restraint, and the volume change upon martensite transformation.        
These problems occur when welding martensitic steels regardless of the prior condition, whether annealed, hardened, or hardened-and-tempered. They can occur with all types of welding, including GTAW, GMAW, laser-beam, friction, resistance and electron-beam. In all cases, the high-temperature HAZ will be in the “as-quenched” condition after welding. Any mechanical straining after welding (i.e. continuous tube mill forming/straightening) will cause the martensitic HAZ to crack. Conventional processes such as batch pre-heating and post weld heat treating (PWHT) do not lend themselves to cost-efficient, high-quality, high volume production.
In a minority of applications for air hardenable steel, welding is used to join separate pieces of the material. For these applications, textbooks related to the field teach a “preheating” method to control cracking. Using the preheating method, each entire workpiece is heated prior to welding. The latent heat in the workpiece reduces the cooling rate of the welded seam, and cracking is thus inhibited. However, there is no indication in the prior art that the preheating method was successfully applied to seam-welding roller-formed air hardenable steel into a tube—as can be seen by the fact that such a tube is not available. This may indicate limitations inherent in the method of preheating—for example, the method may only be reliable for relatively short welds at low welding speeds (i.e. manual welding) joining relatively small work pieces, where accumulated stresses due to weld shrinkage are relatively low and significant cooling of the work pieces does not occur before the weld bead is run from one end to the other. Or, it may indicate that others in the industry did not envision applying the method to the production of seam-welded air hardenable steel tubes, perhaps because they did not envision the tremendous utility of this type of tubing.
The use of air hardenable steels for structural applications was introduced in U.S. Pat. No. 5,485,948 and further described in U.S. Pat. No. 5,871,140. These patents provide brazed structures that take advantage of the fact that air hardenable stainless steel can be simultaneously brazed and hardened in one heat treating operation, including structures containing drawn tubing. Use of a lock seam tubing brazed and hardened in this manner was seen as having advantages in producing load bearing structures at low cost. However, the proposed lock seam increases the weight of the tubing, creates a stress riser and an uneven exterior and/or interior surface, and causes the tube to bow or distort when it is heat-treated.
Drawn air hardenable stainless steel tubing is found in the prior art and is available for purchase; however, it is prohibitively expensive for most structural applications.
Due to the performance liabilities of lock seam air hardenable stainless steel tubing and the expense of drawn air hardenable stainless steel tubing, it was determined that seam-welded air hardenable stainless steel tubing would give the highest overall performance for most structural applications. But, it was found that seam-welded air hardenable steel tubing, of either the preferred stainless type or of the non-stainless type, was not available. No one had solved the technical challenge of producing seam-welded tubing from air hardenable steel strip such that the tubing's HAZ is substantially free of cracks. Accordingly, the prior art did not provide a seam-welded, air hardenable steel tube.
Other than U.S. Pat. Nos. 5,485,948 and 5,871,140 and related international filings, the prior art did not describe the use of air hardenable stainless steel for structural purposes, although this is not surprising due to the difficulties involved in welding this material as described herein above. The most common structural materials in use today include reinforced concrete, mild steel, high strength steels, aluminum alloys, woods, and exotic materials such as carbon composites and titanium. Specific application requirements govern the selection of a structural material and the engineer chooses on the basis of factors such as cost, durability, corrosion-resistance, strength-to-weight ratio and stiffness-to-weight ratio, among other properties. Unfortunately, the choice often requires the engineer, and ultimately the end users of the structure, to sacrifice one or more desirable features, such as low cost, safety, lightness, or durability.
Structural materials are currently available in a broad range of strength-to-weight ratios, or specific strengths, but the costs of these materials generally increase disproportionately to their specific strengths. Carbon composites and titanium, for example, while being perhaps ten times stronger than mild steel for a given weight, are typically more than fifty times more expensive when used to bear a given load. Consequently, such high performance materials are typically used only in on small items, such as bicycles and tennis racquets, or in applications where the high cost is justified, such as in aircraft.
In cost-sensitive applications such as automobiles, conventional engineering materials force a trade-off between cost and fuel efficiency, safety, and performance. Consequently, the typical vehicle tends to have a frame that is both too heavy and too weak. A heavy frame requires a more powerful propulsion system, which leads to higher fuel consumption, higher emissions, and higher maintenance costs. The more powerful propulsion system is itself more expensive to build, uses more material, requires more energy to produce and leads to more emissions related to its manufacture. A lightweight, weak frame compromises the durability of the vehicle and the safety of its occupants.
Lack of a strong and lightweight yet low cost structural material is a main hindrance to the development of economically viable low emissions vehicles; vehicles that can compare in performance, safety, comfort, and price to those powered by the typical internal combustion power system. Without a light, economically competitive structural material to enable alternative power systems for moving vehicles, drastic emissions reduction will be extremely difficult to realize. Current lightweight alternatives to common steel for vehicle bodies are aluminum, plastics, high-strength steels, and exotic materials like magnesium and titanium. None of these materials can offer the required set of performance parameters for the frame elements of a vehicle, at a sufficiently low price. Some of these materials are superior to mild steel in one respect or another, but their cost offsets the advantage. In other respects, their performance is worse than mild steel.
With no lightweight structural material offering a quantum overall advancement in performance and price over mild steel, design of a lighter vehicle is an exercise in balancing trade-offs, offsetting disadvantages as well as possible. For example, vehicle lightness and safety are traditionally inversely related. In a 1997 article that surveys the currently available lightweight alternatives for vehicle bodies, two MIT professors wrote, “a lightweight car cannot rely on its structural components to protect passengers in the event of a crash and so will need to employ additional systems, like air bags, which add some weight.” With the current choice of structural materials, it is likely that lightweight vehicle body research and development will be a lengthy and expensive process, with no certainty of reaching the performance and cost targets.
Compromises between objectives, brought about by structural material limitations, can be seen in many other areas and are being found increasingly unacceptable. For example, reinforced concrete bridges are weak and heavy, subject to failure in earthquakes and susceptible to aging. They also must be built on-site and take long amounts of time to build, which means new bridge construction undertaken to alleviate traffic congestion aggravates the problem during the lengthy construction period. Alternative bridge-building materials, such as carbon composites, are much stronger than reinforced concrete, and bridges of these materials can be erected faster than their concrete counterparts, but these materials are prohibitively expensive.
Solving traffic congestion problems is also dependent upon the development of a structural material that will deliver the required performance characteristics at a supportable cost. In many urban areas, the cost of expanding highways and freeways is prohibitive. Expanding vertically, building elevated inter-city high-speed trains and elevated “double-decker” freeways, is often proposed, but subsequent calculations of construction costs for the elevated railway or roadbed prohibit widespread adoption of these space-efficient solutions.
In building construction, the advantages offered by steel framing, of the type described in the prior art, are offset by its increased cost over traditional structural materials such as wood and masonry. A conventional steel-framed building is safer, more durable, and more energy-efficient than a comparably sized wood-framed building; but also more expensive.
Compromises between cost and performance can be seen in many other structural applications, such as aircraft, ships, bicycles, fluid and gas transfer piping, and heat exchangers. If seam-welded air hardenable steel tubing could be created, and specifically if it could be created through an inexpensive process using stainless steel, it could provide structural characteristics equivalent or superior to much more expensive materials.